Help! Y Do Men Like Women Who Cheat?

I recently made contact with an old childhood friend, and have come to realize that we enjoy each other's company. unfortunately, he's been dating someone for almost 3 years now, and the girl cheats on him (openly), her family's always harassing him for money & stuff, and she gets particularly vicious if she sees another woman acting too familiar with her man.

what i really don't understand is, he keeps making excuses for her, and m wondering if i shouldn't just write him off and get on with my life.